The recycling of copper from copper-containing scrap accounts for a significant part of the United States copper supply. While scrap with a copper concentration greater than 50% may be economically smelted, lower grades of copper scrap, particularly those with a copper concentration of less than 30% by weight, are infrequently recycled because existing methods are not economical. Low grade copper scrap is generated at a rate of 200,000-300,000 tons per year. An example of low grade scrap is the "breakage" portion of shredded automobile scrap produced by the sorting of copper-containing motors. The "breakage" typically contains 10-20% copper by weight.
Recycled copper can be used in the production of valuable copper compounds. Copper compounds such as tribasic copper sulfate (3Cu(OH).sub.2 .multidot.CuSO.sub.4) and copper sulfate (CuSO.sub.4) have wide application. Copper sulfate is a fungicide and algicide, and is used as a source of copper in animal nutrition, as a fertilizer, and as a source for other copper compounds. Tribasic copper sulfate is a fungicide and is a source for other copper compounds.
In view of the large quantity of low grade copper scrap and the commercial value of copper compounds, an economical method for the production of copper compounds from low grade copper materials is needed. Moreover, it would be beneficial to have a method which provides copper compounds in a concentrated form with minimal contamination by other metals.
To be economical, the method should exclude expensive, intermediary steps. It would be beneficial if the copper could be recovered from a copper-bearing leachate in the form of a material that is readily converted into other commercially valuable copper compounds, without the necessity of an expensive intermediary copper purification step.
To produce copper compounds in a concentrated form with minimal contamination, the method of recovery of copper from the copper-containing leachate should be specific for copper and exclude contaminating metals, such as zinc and nickel.
U.S. Pat. No. 3,971,652 (1976) by Bryson discloses the use of an ammoniated ammonium carbonate solution containing sulfate ions to extract copper from dross recovered from lead smelting. The copper was separated from the leachate by contacting the leachate with an organic solvent containing an hydroxy-oxime (see, for example, U.S. Pat. No. 3,244,873 to Swanson (1965)) which is a specific extractant for copper. Copper in the organic phase was then back extracted into a strong aqueous mineral acid solution such as sulfuric acid. The copper could then be purified from acidic copper sulfate solution by electrolysis or reduction with a reducing agent. Alternately, copper sulfate could be produced from the acidic copper sulfate solution by evaporation of the solution. The Swanson method has the drawback of the use of expensive organic solvents.
Copper ammine sulfate has long been regarded as a desirable lixiviant for the dissolution of copper from copper-containing sources. In "Extractive Metallurgy of Copper", Arbiter & Milligan eds., Vol. 2, pp. 974-993 (1976), a method is described for the treatment of a copper tetrammine sulfate leachate with sulfur dioxide for the precipitation of intermediate sulfites, preferably cuprous ammonium sulfite, Cu.sub.2 SO.sub.3 (NH.sub.4).sub.2 SO.sub.3. The precipitate is washed ,and thermally treated to decompose to copper metal of high purity. This method, however, does not provide for the production of copper compounds directly from copper ammine sulfate leachate.
Accordingly, there is a need for a process which provides for the economical recovery of copper from copper-containing scrap, especially scrap having less than about 30% copper, whereby copper is recovered in a form that is readily converted into other commercially valuable copper compounds, without the necessity of an expensive intermediary step of copper purification. The method should provide for the recovery of copper with a minimal amount of contaminating metals, such as zinc and nickel.
It has now been found that a mixture of copper-bearing compositions, and in particular, the mono- and trihydrates of cupric tetrammine sulfate (HCTS) having the formulas Cu(NH.sub.3).sub.4 SO.sub.4 .multidot.H.sub.2 O and Cu(NH.sub.3).sub.4 SO.sub.4 .multidot.3H.sub.2 O, respectively, can be selectively precipitated from a copper ammine sulfate solution. The hydrates of copper tetrammine sulfate are known compounds which were known to be formed by the dehydration of cupric tetrammine solution (see, e.g., Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, pp. 251-260 (1960)). The selectivity in precipitation made possible by the practice of the present invention allows for the isolation of copper in the precipitate, leaving nickel and zinc ammine sulfates in solution. The precipitated HCTS, typically having less than a total of 0.02% by weight of nickel and zinc contamination, can be readily converted into high purity copper compounds.